The art of lithographic printing is based on the immiscibility of ink and water. A lithographic printing plate is composed of ink receptive regions, commonly referred to as the “image area,” and hydrophilic regions. When the surface of the printing plate is moistened with water and printing ink is applied, the hydrophilic regions retain the water and repel the printing ink, and the image area retains the printing ink and repels the water. The printing ink retained on the image area may then be transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the desired surface.
Lithographic printing plates precursors typically include a radiation-sensitive coating applied over the hydrophilic surface of a substrate. Conventional radiation-sensitive coatings include photosensitive components dispersed within an organic polymeric binder. After a portion of the coating is exposed to radiation (commonly referred to as imagewise exposure), the exposed portion becomes either more developable or less developable in a particular liquid than an unexposed portion of the coating. A printing plate precursor is generally considered a positive-working plate if, after exposure to radiation, the exposed portions or areas of the radiation-sensitive coating become more developable and are removed in the developing process to reveal the hydrophilic surface. Conversely, the precursor is considered a negative-working plate if the exposed portions or areas become less developable in the developer and the unexposed portions or areas are removed in the developing process.
After imaging, the precursors are contacted with a developer to remove either the exposed or unexposed portions of the radiation-sensitive coating to form a printing plate. This process is generally performed using a developer system, which is capable of receiving imaged printing plate precursors, and then contacting the precursors with a developer either by spraying the developer onto the precursor or by immersing the precursor in a developer bath. During development, portions of the radiation-sensitive composition are removed from the precursor to reveal the substrate surface, and portions remain on the plate to provide an ink-receptive image. After the developing step, the printing plates are then washed to remove excess developer.
Suitable developers for processing printing plate precursors may fall within at least three general categories defined by the developer's pH range and whether the developer includes an organic solvent and/or dispersing agent. Each category is effective in developing particular types of radiation-sensitive compositions. A first category of developers includes highly alkaline aqueous developers, generally having a pH of greater than about 13. These developers utilize the presence of hydroxyl ions to develop the imaged printing plate precursors. However, these highly alkaline developers do not normally include organic solvents (e.g. organic alcohols) because the combined activities of the hydroxyl ions and organic solvent may degrade the image area on a printing plate precursor. Examples of developers falling within this category include ProTherm brand developers and MX 1813 brand developers, both available from Kodak Polychrome Graphics, Norwalk, Conn.
A second category of developers includes acidic to substantially neutral developers, generally having a pH between about 2 and less than 8. Developers falling within this second category contain organic solvents, acids and/or weak bases to control pH activity, and dispersing agents (e.g. organic sulfates or sulfonates) to suspend, disperse or dissolve printing plate coating materials removed during the development process. These types of developers do not include strong bases. An example of a developer falling within this category is the Aqua-Image brand developer available from Kodak Polychrome Graphics.
A third category includes developers that have pH ranges between about 8 and less than about 13, more particularly between about 8 and about 12. These developers may contain organic solvents, dispersing agents and at least one weak base (e.g., an organic amine such as ethanolamine, diethanolamine or triethanolamine). An example of a developer falling within this category includes 956 brand developer available from Kodak Polychrome Graphics.
During the development of printing plate precursors, the activity of a volume of developer (i.e. the ability of the developer to remove desired portions of the radiation-sensitive coating to produce an image) may vary due to the depletion of, or changes in, various components of the developer. For example, over time, the concentration of the organic solvent or the dispersing agent, as well as the pH of the developer may vary. Changes in developer activity may be caused by loss of developer components as printing plates carry developer out of the developer system after development. Additionally, interactions with the radiation-sensitive coatings of the printing plate precursors may also affect developer activity. In particular, the pH of the developer may change due to acid/base interactions with imaged portions of radiation-sensitive coatings. This loss of developer activity may result in inconsistency in overall dot density over a cycle of developed printing plates, which may have adverse effects during printing.
Thus, in certain circumstances, developer activity is been restored, maintained or increased (collectively referred to herein as “maintained”) through the addition of replenishers or regenerators to the volume of developer. As used herein, the term “replenisher” refers to a substance having approximately the same activity as the developer. Thus, the developer itself is often used as a replenisher to maintain the volume of developer contained in the developer apparatus. As used herein, the term “regenerator” refers to a substance having a different level of activity than the developer to which it is added. For example, the regenerator may have a different pH than the developer.
Examples of first category (i.e. highly alkaline) developer regenerators include 9008 brand regenerator and MX 1919 brand regenerator, both available from Kodak Polychrome Graphics. These regenerators include the same active components as the developer, but have a higher pH in order to maintain developer activity. An example of a second category (acidic to neutral) developer regenerator includes Aqua-Image Top-Off brand regenerator, also available from Kodak Polychrome Graphics. This regenerator includes a higher concentration of organic solvent and dispersing agent than the developer to maintain activity.
Traditionally, the third category of developers (i.e. containing a weak base and having a pH between about 8 and less than about 13) did not require the use of a regenerator because precise control of developer activity was not thought necessary to develop a standard cycle of printing plate precursors with consistent dot densities. However, recent advances in imaging technology, as well as a desire to utilize longer development cycles, have necessitated an increasing level of precision and consistency in dot density for plates developed by third category developers. For example, the advent of laser imaging systems has provided for more precise imaging than conventional imaging methods. This enhanced imaging precision, in turn, has led to an increase in the use of stochastic (FM) screening methods in the printing industry. Stochastic screening varies spacing between dots (and optionally dot size) to create the illusion of continuous tones. In contrast, conventional screening (AM) methods rely on changes in dot size to produce the illusion of continuous tones. First order stochastic screening produces dots having the same size, but varies spacing between the dots. Second order screening produces dots with both size and spacing variation. Hybrid screening uses a combination of conventional and stochastic screening.
Stochastic screening requires precisely controlled dot size, density and spacing in order to create the appearance of continuous tones. However, the activity of third category developers changes sufficiently during the development of a cycle of printing plate precursors to adversely affect dot density consistency, particularly over extended development cycles.